We are evaluating the mechanism of the blood pressure response induced by osmotic and nonosmotic stimuli of arginine vasopressin (AVP). All experiments are carried out in awake, unrestrained, bilaterally nephrectomized rats. Hypertonic saline administered to the Long Evans strain induced a significant increase in mean blood pressure which was not observed in the Brattleboro strain of rat. Naloxone, an opiate antagonist, had no effect on this rise in blood pressure in the Long Evans rat. In the Brattleboro rats peritoneal dialysis with a 25% mannitol/saline solution induced an initial fall in blood pressure with a gradual return to baseline. This manipulation in the long Evans animals produced a significant and progressive increase in blood pressure over the two hour observation period. Thus these two hypertensive maneuvers require the pressence of arginine vasopressin.